The free version of Google maps API makes http request which will be treated as non-secure item by the browser. Browser may show security warning like "Do you want to view only the webpage content that was delivered securely?" with yes and no buttons. Obvioulsy, this is not only irritating for the users but can affect user experience with website in case they choose incorrect option.
To avoid this, use premier version of Google maps.
You use licensed version of Google maps.
You get a client id which you will be using like key in free version of google maps. But unlike the keys, client id is more secure as Google authorize the URLs that you want to use client id. You can register any number of URLs with same client id. You can even track some data using it.
By the way, if your web application is for some commercial website (i.e. if your site is only available to paying customers or if your site is only accessible within your company or on your intranet), then also you will have to use this premier version.